Well, here goes. I'm going to document what it has taken me to get the MCE
component of Vista working on my PC and on my XBOX 360.

Step 1.
Change to domain.
Step 2.
Add Library to Media Center on Domain Share.
Step 3.
Add XBOX 360 as MCX.
Step 4.
Watch Step 3 fail over and over again no matter what I do, even going so far
as to undo all the things required for internet access on a marvell ethernet
adapter. Finally figure out it is the domain causing the problem, and set
everything back the way it was before Step 4 (including deleting the
Extendered in Media Center, disconnecting from the PC in the XBOX 360
dashboard, and restarting the XBOX 360).
Step 5.
Rejoin WORKGROUP, leaving the domain.
Step 6.
Start Media Center and add XBOX 360.
Step 7.
Access files on Vista PC using XBOX 360 as Extender.
Step 8.
Rejoin Domain.
Step 9.
Access files on Vista PC using XBOX 360 as Extender. WHOOHOO!
Step 10.
Try to access all my media that is on the network shares from the XBOX 360.
Realize that the XBOX 360 cannot access the network shares and understanding
the implications behind all that. Turn off XBOX 360.
Step 11.
Looked at some stuff on the web required Visual C and SDK's and saying
F-That. Looked some more and found info on using the profile to map a drive.
It's insecure as hell, but it works and I've got a small network, so I went
for it.
Step 12.
Started my trusty friend, Notepad. Wrote a net use script and saved it as
mcx.bat in the newly created c:\netlogon folder on the Vista PC. Here's what
it had (edited for names and passwords of course) without the _ at the front
and end:
_net use z: \\manhattan\D /user:"ABOMB\Bob Oppenheimer" "proton"_
Where proton is the password for Bob Oppenheimer who is a user on the domain
called ABOMB with rights to the files and folders I want on share D of server
manhattan.
Step 13.
I shared the c:\netlogon folder as netlogon on the Vista Pc. Right click on
the folder, left click properties, click the sharing tab. Click the Share
Button. To the left of the add button, drop down the list and select find.
I added domain users and MCX1 as readers. Local Administrators were already
owners. Click Share. Click Done. Click security. Click Edit. I added
Domain Users and MCX1 with read & execute privileges just to be share. Click
OK and Click OK or Close.
Step 14.
With elevated privileges I ran Computer Management (I was logged on as
administrator on the Vista PC so i just clicked the start orb, right clicked
Computer, and left clicked Manage, you could click the start orb, type
"compmgmt.msc" without the quotes, right click on compmgmt.msc that appeared
at the top under programs and left click run as administrator).
Step 15.
In Computer Management, go to Systems Tools, and then go to Local Users and
Groups. Click on Users. Right Click on MCX1 and left click on properties.
Click the Profile Tab. Under Logon Script type MCX.bat. Click Ok. Close
Computer Management.
Step 16.
Start XBOX 360. Log on to Media Center from XBOX 360. Go to Settings. Go
to Library. Voila. My folders are there and I add them. Then I just had to
wait for the library to build. My XBOX 360 as a Media Center Extender is
playing songs from my domain network share on a server right now.

That's it. You should be able to replicate the same thing and/or modify it
for different situations.

A few comments:

Microsoft should add the ability for Media Center Extenders to be operated
more securely and easily in a domain. I've been in quite a few larger
offices that could really benefit by having a single media center server and
extenders at the different projector and flat panel locations. The easiet
way I could see would be for Media Center to detect when it is in a domain
and then ask for a pre-created domain user to be issued to each newly created
extender, instead of just incrementing MCX on the local computer. This could
allow regular Active Directory and centralized management. Is there already
a media server I don't know about? Even if there is, this method would
surely have to be cheaper. Logon to the Vista Media Center PC as a Domain
Administrator, Start Media Center, add the extender, it asks for a user and
runs you through the regular Select Users, Computers, or Groups dialog
(except it only lets you add users) for the domain, you pick the user and
you're done.

The method that I documented above these comments is not a secure
implementation of network shares in an extender. The server, share name,
domain, domain user and domain user password are all transmitted in clear
text across the network to the server as well as saved in a clear text file
on the PC. Your best bet to shore this up as tightly as possible is to
restrict the permissions on the netlogon folder a lot tighter until you break
it, and then loosen up one until it works again. You should also create a
special user for access to those shares and those shares only in the domain.
The user used in the MCX.bat script should not be a regular domain user, but
one that has no access to anything but those shares. I haven't tried this,
so I don't know if it's possible, but you could possibly restrict permissions
to read&execute only on the shares and see if the extender will let you get
away with it. That will keep the extender from being able to delete files.

Hope this helps